Wildlife and Terrestrial Ecosystems
Contact Information
  • Wildlife and Terrestrial Ecosystems
  • Southwest Forest Science Complex
  • 2500 South Pine Knoll
  • Flagstaff, AZ 86001-6381
  • (928) 556-2001
You are here: WTE Home / Invasive Species / Invasive Species Research Model

Invasive Species Research Model

Problem Statement

Exotic species invasions threaten the ecological integrity and biological diversity of native ecosystems around the world. These invasions cost an estimated 137 billion dollars each year in the US alone. There is a critical need to understand the processes underlying exotic species invasions and to evaluate the efficacy of the tools currently used in battling these invasions to determine when and where these tools will be most effective.

Invasive species research model for weed invasions
Invasive species research model for weed invasions — This model illustrates how invasions differ from natural disturbance in that they shift a native system off of its dynamic equilibrium (a) into a new trajectory (b) that may not reach a new equilibrium for some time. The result is a trajectory that can be altered, but not stopped with powerful management tools. Although invasives are often managed by combining methods in integrated pest management, to simplify, this model focuses on herbicide and biological control as primary tools with independent trajectories.

Research Approach

Our strategy is to understand exotic species invasions in the context of disturbance ecology. Within this framework, native systems are thought to persist around a stable equilibrium state. Although disturbances like fire, drought, etc. can shift systems off this equilibrium, successional processes immediately begin to move them back toward the equilibrium. When exotic species invade, they can alter this dynamic. If the invader is very mild mannered, the system may rapidly resume a new equilibrium state so similar to the original that we do not notice the change. However, if the invader is particularly nasty, it can actually launch a new trajectory that may not reach a new equilibrium for hundreds of years. For example, as cheatgrass invades the Great Basin it increases fire frequency. This in turn favors more cheatgrass invasion, which further increases fire frequency. Clearly this system is on a new trajectory, but it is not clear when or where it will settle into a new equilibrium. Recognizing that exotic species invasions launch systems onto new trajectories toward new equilibrium states is a critical step in understanding and managing invasions. Equally important is the recognition that our management tools rarely restore invaded systems to their former state (Pearson and Ortega 2009). Management does not stop invasions, it alters the trajectory. Therefore, it is critical to ensure that any new trajectories we initiate improve on the invasion trajectory. From a management standpoint, the unaltered invasion trajectory is the no-action alternative, and management strategies represent alternative actions that alter these trajectories. Therefore, the first step in addressing invasions is to understand the invasion trajectory, or no-action alternative, to better understand the mechanisms underlying invasions and their impacts. With this baseline information, we are better positioned to effectively apply management and also evaluate the outcome relative to the invasion trajectory. By developing our research along this framework with studies addressing each trajectory, we are providing managers with critical information to make defensible decisions to effectively apply the available tools.


We have applied this framework in the context of spotted knapweed invasions to expand our understandings of the no action invasion trajectory and evaluate the efficacy of management alternatives like herbicides and biological control agents. This work has contributed to understandings of invasion ecology, invasion impacts on native plants and animals, and the efficacy of herbicides and biological control agents.


  • University of Montana
  • State and Private Forestry
  • Montana Department of Natural Resources and Conservation
  • Montana Fish Wildlife and Parks
  • City of Missoula
  • Bitterroot National Forest
  • Lolo National Forest
  • Confederated Salish and Kootenai Tribes
  • Plum Creek Timber Company